I have an application where in the ULN2003 has to drive IR Leds and let a surge current through the IR led ! The high frequency that the uln will have to switch is between 100Khz to 50Khz (10uS to 20uS pulse width). How much voltage voltage loss can be see through the ULN2003. The uln will be given a Vcc of 3.3V and will also have input logic 1 = 3.3V.

No such information is given in the datasheet apart from Toff = Ton = 1uS Max

According to the datasheet at 1.5V I get 100mA ( which is the current I require ) and thus for 2 LED's drop is 3V, so if there would have been no losses in ULN the remaining 0.3V/100mA would give me 3ohm current limiting resistance !
But that was assuming no switching loss , all depends on ULN2003 switching loss

Well I am little confused now ! while designing the PCb i had enquired with many ppl who have worked on this but every used to say 'Assuming no losses' the only difference between them and me being 3.3V and they working at 5V ! but even in a 5V application more than 20% loss cannot be accounted for assuming no loss ! Could you help me out ? Also SFH4503 takes 1.5V and not 1.8V as suggested ( 1.8V is when u keep it on for 20ms and the junction temperature increases and thus increases the junction voltage, here i plan not more than 20uS ) So again the question comes back to voltage drop across ULN2003 at 50Khz to 100Khz

You may not be able to use a ULN2003 at that high of a frequency. Internally, it's seven Darlington pairs. Darlington pairs provide high gain, but are very slow when compared to discrete transistors.

Why don't you try connecting up a ULN2003 to a 100kHz square wave like you plan on driving it with, and a pull-up resistor on an output to 3.3v? A wild guess suggests that you will observe about 0.9v at the collector of an output with a 100mA load, or 24 Ohms to 3.3v. Your mileage may vary significantly.

Instead of using a Darlington driver, you might go with discrete transistors like a BC547 or 2N2222 with a 470 Ohm resistor on the base. That way you'll have a good deal more bandwidth, and the saturation voltage may get down around 0.2v.

You need logic level MOSFETs. Some are specified for Vgs as low as 1.5V. With 3.3V of gate drive, you can get Rds(on) as low as 40 milliohms. This would give you a saturation voltage less than 4mV with 100mA through it.
Example: http://www.diodes.com/datasheets/ZXMN2A02N8.pdf